1. Field of the Invention
The invention relates to a process for the preparation of aluminum salts of cyclic saturated or unsaturated phosphinic acids, in particular aluminum salts of 1-hydroxydihydrophosphole oxides and 1-hydroxyphospholane oxides.
2. Description of the Prior Art
Salts of phosphinic acids have already been known per se for some time and are recommended in particular as flame-inhibiting additives for thermoplastics such as polyesters or polyamides. Thus, DE-A1-2252258 describes alkali metal salts of phosphinic acids. They must, however, be added in relatively large amounts and some have an unfavorable corrosion-promoting influence on the processing machinery.
EP-A3-0699708 discloses polyester molding compositions which contain calcium salts or aluminum salts of phosphinic acids of the following formulae: ##STR1##
wherein
R.sup.1 and R.sup.2 are C.sub.1 -C.sub.6 -alkyl, linear or branched; phenyl; PA1 R.sup.3 is C.sub.1 -C.sub.10 -alkylene, linear or branched; arylene; alkylarylene; or arylalkylene; PA1 M is a calcium ion or aluminum ion; PA1 m is 2 or 3; PA1 n is 1 or 3; and PA1 x is 1or 2.
The preparation of the aluminum salts, described in this European patent application, of the phosphinic acids mentioned therein requires a relatively long reaction time, namely 24, or even 65, hours.
Although a large number of phosphinic acid compounds and preparation processes for these compounds are already known, there is still a need for improved processes for the preparation of suitable phosphinic acid salts which may be used as agents for improving the flame resistance of plastics.
The object of the invention is thus to provide a process in which it is possible to prepare desirable phosphinic acid salts which may be used as flameproofing agents; the object of the invention is also to provide a process which is economical and which leads to high yields in short reaction times. The object of the invention is also to provide a process which is environmentally friendly and which produces no or only small amounts of substances which must be removed during isolation of the final product and be disposed of.